this.tags is on the prototype instead of the element.

[ScarletCarson]

This:

Tagging.prototype = {
        tags: [],
...

Should be:

var Tagging = function(elem, options) {
        this.elem = elem;
        this.$elem = $(elem);
        this.options = options;
        this.tags = [];
}

In order to allow a user to use more than one tagging block per page.

[Stephen Edmondson]

I'm seeing this issue as well. Unable to have multiple instances of inputs on the same page as a result.

[Nasser Al-Wohaibi]

@MSommer MSommer/taggingJS fork solves the issue as suggested here.

[Damian Penney]

Just as a heads up but the @MSommer fork doesn't have the change applied to the minified code, works great though.